Before a pressurized-water nuclear reactor is commissioned, tests are conducted, and during these the primary water circuit is filled at ambient temperature and then at the operating temperature of the reactor. This water is circulated through the entire primary circuit, using the pumps circulating the pressurized water for cooling the reactor. These tests are carried out before the reactor core is loaded and, in particular, make it possible to eliminate all metallic or similar particles which originate from the operations of manufacturing and assembling the entire primary circuit of the reactor and which are still located in this circuit after it has been assembled. In fact, in spite of the great precautions taken during the machining and assembly of the elements of the vessel, it is not possible to prevent the presence in this vessel of small metallic particles which it is absolutely essential to eliminate before the reactor is commissioned.
Filtration devices, through which passes the water circulating in the primary circuit during the tests, particularly during the hot-running tests of the reactor, are therefore arranged in this primary circuit. Such filtration devices make it possible to stop the circulating particles which are retained either on the filter itself or immediately upstream of the filter.
It is known to eliminate the particles of a size larger than 3.times.10.sup.-3 m at the inlet of the core at the level of the porting plate intended for receiving the core assemblies.
French Patent 2,280,178 describes a modular filtration device comprising a support allowing it to be fastened to the lower plate of the core of a pressurized-water nuclear reactor in place of a fuel assembly. This filtration device has a grating fastened to the support so as to be inserted in the circulation path of the water passing through the lower plate of the reactor core via the holes provided in this plate for the passage of the reactor cooling fluid. This grating makes it possible to stop at the entrance of the core the particles which may be transported by the water during the tests. Each of the assembly positions is occupied by a filtration device, with the result that the entire flow of water circulating in the primary circuit during the tests on the reactor is subjected to filtration. The grating of the filtration devices usually consists of a fine-mesh metal gauze made of stainless steel resting on a flat grating, the meshes of which are of larger dimensions than the meshes of the gauze and which has fairly high rigidity because it is produced from a wire of relatively large diameter of the order of 2.times.10.sup.-3 m. Moreover, the circular central part of the grating bears on a yoke integral with the support of the filtration device. It is necessary, in fact, for the grating to be capable of withstanding substantial forces attributed to the very high circulation speed of the fluid during the tests.
However, it has been observed, during the use of such devices, that the gratings undergo considerable wear which can result in their fracture during prolonged tests. In fact, the circulation of water passing through the gratings causes the latter to flap, thus resulting in impacts between the metal gauze and the supporting grating.
These impacts cause considerable wear and fracturing of the gratings.
When a simple metal gauze is used as a filtration element, the mechanical strength of this element is diminished, and wear and breaking of wires of the metal gauze are observed at the central bearing support. This excessive wear is attributed to the friction and fatigue of the wires at the nodes of the network.
On the other hand, the passage cross-section of the flat gratings, which corresponds to the cross-section of the filtration elements, is necessarily reduced, the more so, the finer the mesh of the network and the larger the diameter of the wire constituting the grating.
It has been proposed to increase the passage cross-section of the filtration elements by the use of conical or elliptical gratings, but in this case the directional deflection of the streams of fluid passing through the filtration element is very great, and this is particularly detrimental in the case of modular devices arranged next to one another according to a closely set network corresponding to the network of assemblies of the reactor core. The various streams of fluid passing through the filtration devices then interfere with one another and considerably disturb the circulation of water in the reactor vessel.